Lagerwall and Clark described the surface-stabilized ferroelectric liquid crystal (SSFLC) effect and its application to electro-optic shutters and display devices (U.S. Pat. Nos. 4,367,924 and 4,563,059). In SSFLC cells, the FLC is aligned between transparent electrodes in the so called "bookshelf" alignment in which the smectic layers are substantially perpendicular to the electrodes and the long axis of the FLC molecules is parallel to the electrodes. In this configuration, the natural helix typically formed in the ferroelectric phase is suppressed by surface interactions in the cell. Suppression of the helix results in a bistable cell in which the optic axis of the cell can be rotated in the plane of the electrodes by 2.THETA., where .THETA. is the tilt angle, by changing the sign of the applied driving voltage. Tilt angle is an intrinsic property of a FLC. In order to suppress the helix, the cell thickness (d) must be comparable to or smaller than the magnitude of the pitch of the helix. Thus, for applications in the visible in which cell thicknesses of 0.5-6 .mu.mm are most useful (assuming a birefringence of 0.15-0.3), the chiral tilted smectic ferroelectric natural helix pitch in the FLC should be longer than 0.5-10 .mu.m.
Electro-optic effects in FLC cells in which the helix in the smectic C* phase is not suppressed by surface-stabilization have also been described. The Distorted Helix Ferroelectric (DHF) effect, described for example in Ostovski et al., Advances in Liquid Crystal Research and Applications, Oxford/Budapest. (1980) page 469 and in Funfschilling and Schadt (1989) J. Appl. Phys. 66(8):3877-3882), is observed in FLCs aligned between electrode plates in which the natural helix pitch in the smectic C* (or other chiral tilted smectic ferroelectric) phase is sufficiently tight, i.e., shorter than the FLC cell thickness (d), so that the helix is not suppressed. DHFLC electro-optic devices have an FLC aligned between electrode plates. Most typically the FLC is planar aligned and in the "bookshelf" geometry. A driving voltage applied to the electrodes to generate an electric field across the FLC layer. Unlike, surface-stabilized FLC devices, the natural helix of the aligned chiral smectic phase is present in the aligned FLC material in the DHF effect device. The helix forms parallel to the plates and perpendicular to the smectic layers as illustrated in FIG. 1. The magnitude of the pitch of the helix is the distance along the helix axis for one full turn of the helix and the sign of the pitch (+ or -) represents the direction of twist of the helix. The term "tight" pitch, which can be a positive or negative value, is associated with shorter axial lengths for one full turn of the helix. The term "pitch" as used herein refers to the magnitude of the pitch; the terms "sign of the pitch" or "twist" refer to the direction of twist of the helix.
SSFLC and DHF cells can be operated in reflection mode in which one of the electrode plates is reflective (see, for example, U.S. Pat. No. 4,799,776).
When the magnitude of the smectic C* helical pitch is comparable to the wavelength of visible light, a striped pattern appears in the device and in effect a diffraction grating is formed. If the magnitude of the pitch is less than the wavelength of light (and preferably less than 1/2 .lambda. of light), light diffraction is minimized and the apparent refractive index of the FLC is the average over many director orientation of the helix as shown in FIG. 1. In the field-free state with zero applied electric field and with no surface stabilization, the smectic C* helix is in its natural state. The molecular director, n, makes an angle, .THETA., with the layer normal. In the field-free (E=0) state, due to the presence of the helix, averaging occurs and the apparent optic axis of the DHFLC coincides with the helix axis, as shown in FIG. 1.
If the voltage applied across the FLC layer is above a certain critical level E.sub.c, the helix is completely unwound forming two distinct optical states, as in an SSFLC device. Application of a voltage below E.sub.c deforms the helix, generating an effective rotation of the optic axis of the DHFLC. The orientation of the optic axis of the DHFLC layer can be changed in a continuous fashion proportional to the applied electric field changing the optical anisotropy of the FLC. DHF cells display rotation of their optic axis that is dependent on the magnitude of the applied electric field and also exhibit a change in apparent birefringence (.increment.n) as a function of the magnitude of the applied electric field.
The maximum field-induced angle of rotation of the optic axis of the DHFLC is .THETA., the tilt angle of the material. A maximum field induced optic axis rotation of 2.THETA. can be obtained by application of a +/-voltage step, +/-E.sub.max, where E.sub.max is the minimum voltage required to obtain a rotation of .THETA. and the magnitude of E.sub.max is less than E.sub.c.
DHF effect cells typically exhibit significantly lower apparent refractive index than SSFLC cells due to the averaging noted above. Thus, for a given desired optical retardation, DHF cells are typically thicker than comparable SSFLC cells. Birefringence for DHFLC cells typically ranges from about 0.06 to 0.13, about 1/2 that of SSFLC cells. DHFLC waveplates are as a consequence, typically, thicker than comparable SSFLC waveplates. High birefringence materials are useful in DHF applications to minimize cell thicknesses.
E.sub.c is inversely proportional to the spontaneous polarization of the FLC and the ferroelectric phase pitch, having the relationship: ##EQU1## Thus, the higher the spontaneous polarization and longer than pitch, the lower the voltage necessary to control the effect. Response time (.tau.) for the DHFLC cell is defined as: ##EQU2## where .gamma. is the orientational viscosity and .THETA. is the tilt angle. Increasing P.sub.s lowers the threshold voltage, but does not increase the speed, while tightening the pitch increases both the speed and E.sub.c. By increasing both P.sub.s and decreasing p, the response speed can be significantly increased while maintaining a low threshold voltage. Decreasing the viscosity also improves the response time.
EP Application 309,774 refers to DHFLC display cells with chiral smectic FLCs whose helical structure can be changed by an electric field to to alter the optical anisotropy of the cells. The application refers to DHF cells in which the ratio of the cell thickness to the helix pitch of the FLC (d/p) is more than 5 and preferably more than 10, in which .THETA. is between 22.5.degree. and 50.degree. and in which a so-called phase factor: ##EQU3## is apparently constrained to be greater than 0.1, and preferred to be greater than 0.45. This requirement imposes limits on cell thicknesses which can limit the applications of the DHFLC devices. The application refers to FLC mixtures of 5-alkyl-2-(p-alkoxyphenyl)pyrimidines and a chiral nonracemic terphenyl diester. See also EP application 339,414.
EP application 405,346 refers to bistable FLC cells having aligned FLCs with an achiral smectic C host mixture and a dopant which induces a pitch less than 1 .mu.m. The mixtures are referred to as having spontaneous polarization greater than 10 nC/cm.sup.2 and .THETA. greater than 10.degree.. The described cell displays dark parallel lines at zero voltage between crossed polarizers indicating a non-homogeneous structure.
EP application 404,081 refers to FLC elements having high polarization and tight pitch. This application refers to the use of tight C* pitch materials, where p is at least less than 1/2 d, in SSFLC cells to eliminate optical hysteresis that is observed in cells having high spontaneous polarization. Mixtures having C* pitch in the range 0.25 .mu.m to 0.63 .mu.m were reported, but of these the tightest pitch at room temperature was about 0.39 .mu.m. The tight C* pitch FLC mixtures were reported to have N* pitch at least greater than 8 .mu.m.
Funfshilling and Schadt (1989) J. Appl. Phys. 66(8):3877-3882 refer to fast response, multiplexible DHFLC displays. The authors report that DHF cells require both very short pitch FLCs, with pitch much shorter than cell thickness, and weak surface interactions in the cell. Several methods of cell preparation are reported to decrease the tendency of the helix to unwind: application of shear, the use of different rubbing directions on top and bottom cell plates and surface treatments that lead to zig-zag defects in SSFLCs. These treatments, however, have a detrimental effect on the optical contrast of the cell. They report DHF cells produced by uni-directional rubbing of alignment layers with contrast ratio (ON/OFF) of 12:1 and DHF cells of sheared cells of 40:1. Thus, the production of high contrast DHFLC electro-optic devices is problematic.